1. Field of the Invention
The present invention relates to a fixing device which fixes an image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
In recent years, demand for energy conservation and increase in processing speed has been increasing in the market for image forming apparatuses, such as printers, copiers, and facsimile machines.
In such an image forming apparatus, an unfixed toner image is formed on a recording medium, such as a recording medium sheet, a print sheet, a photosensitive sheet, or an electrostatic recording sheet, through an image forming process based on, for example, electrophotographic, electrostatic, or magnetic recording in accordance with an image transfer method or a direct image formation method. As a fixing device for fixing the unfixed toner image on the recording medium, a fixing device employing a contact heating method, such as a heat roller method, a film heating method, or an electromagnetic induction heating method is widely employed.
Such a fixing device includes, for example, a fixing device employing a belt fixing method and a fixing device employing a surface rapid fixing (SURF) method using a ceramic heater, i.e., a film fixing method.
In recent years, a reduction in warm-up time and first-print time has been demanded of the fixing device employing the belt fixing method (hereinafter referred to as the first issue). The warm-up time refers to the time taken to raise the temperature from a normal temperature to a predetermined reload temperature allowing printing when, for example, power is turned on. The first-print time refers to the time from the reception of a print request to the completion of a sheet discharging operation followed by a print preparatory operation and a printing operation.
Further, along with the increase in processing speed of the image forming apparatus, the number of sheets fed per unit time is increased, and the necessary heat amount is increased. As a result, a so-called temperature drop, i.e., a shortage of heat occurs at the beginning of continuous printing (hereinafter referred to as the second issue).
Meanwhile, in the fixing device employing the SURF method using a ceramic heater, a reduction in heat capacity and device size is achievable, as compared with the fixing device employing the belt fixing method. Accordingly, the above-described first issue is well addressed by the SURF method. The SURF method, however, locally heats a nip portion of a belt, and does not heat the remaining portion of the belt. At a location such as the entrance of the nip portion for receiving the recording medium, therefore, the belt temperature is substantially low, and a fixing failure tends to occur. The fixing failure tends to occur particularly in high-speed image forming apparatuses, in which the belt rotation speed is relatively high and the amount of heat discharged from the belt is increased in the remaining portion other than the nip portion (hereinafter referred to as the third issue).
To address the above-described first to third issues, background fixing devices using an endless belt are configured to obtain favorable fixing performance even when installed in a high-speed image forming apparatus.
As illustrated in FIG. 1, the background fixing device includes an endless belt 100, a metal heat conductor 200, a heat source 300, and a pressure roller 400. The metal heat conductor 200 is formed into a pipe shape, and is disposed inside the endless belt 100. The heat source 300 is disposed inside the metal heat conductor 200. The pressure roller 400 is in contact with the metal heat conductor 200 via the endless belt 100 to form a nip portion N. The endless belt 100 is rotated by the rotation of the pressure roller 400. In this process, the metal heat conductor 200 guides the movement of the endless belt 100. Further, the endless belt 100 is heated, via the metal heat conductor 200, by the heat source 300 inside the metal heat conductor 200. Thereby, the entire endless belt 100 is heated. Accordingly, the first-print time following a heating standby time is reduced, and the shortage of heat in high-speed belt rotation is minimized.
To achieve further energy conservation and reduction in first-print time, it is desired to improve the thermal efficiency of the heating unit. In view of this, the fixing device may also be configured not to indirectly heat the endless belt 100 via the metal heat conductor 200, but to directly heat the endless belt 100 without using the metal heat conductor 200.
As illustrated in FIG. 2, in this configuration, the pipe-shaped metal heat conductor 200 is removed from the inside of the endless belt 100, and is replaced by a plate-shaped nip forming member 500 provided at a position facing the pressure roller 400. In this configuration, a portion of the endless belt 100 other than a portion of the endless belt 100 contacting the nip forming member 500 is directly heated by the heat source 300, thereby substantially improving heat transfer efficiency and reducing power consumption. Accordingly, the first-print time following the heating standby time is further reduced, and moreover a reduction in cost due to the absence of the metal heat conductor 200 can be expected.
A fixing device using an endless belt, such as the above-described fixing device, normally includes restricting members which restrict belt walk in the axial direction thereof. For example, as illustrated in FIG. 3, the background fixing device may include fixing flanges 600 which rotatably hold opposite end portions of the endless belt 100 and restrict belt walk in the axial direction thereof. The background fixing device further includes, between the fixing flanges 600 and the end portions of the endless belt 100, driven rings 700 serving as protecting members for protecting the end portions of the endless belt 100. If the endless belt 100 is subjected to a force exerted in the axial direction thereof and walks toward one side, an end portion of the endless belt 100 hits against the corresponding one of the driven rings 700, and the driven ring 700 rotates together with the endless belt 100. Thereby, friction between the end portion of the endless belt 100 and the flange 600 is prevented.
FIG. 4 is a cross-sectional side view of the background fixing device illustrated in FIG. 3. As illustrated in FIG. 4, in the nip portion N, the endless belt 100 is pressed toward the inner diameter thereof by the pressure roller 400, and is recessed inward from an inner circumferential surface 700a of the driven ring 700. If the endless belt 100 walks, therefore, an end portion of the endless belt 100 comes into sliding contact with an edge E of the inner circumferential surface 700a of the driven ring 700 in the area in which the endless belt 100 is recessed inward from the inner circumferential surface 700a of the driven ring 700, as illustrated in FIG. 5. As a result, the end portion of the endless belt 100 is damaged by stress concentrated thereon owing to the sliding contact.
Further, the endless belt tends to be formed with a reduced thickness to meet the demand in recent years for energy conservation and reduction in first-print time. As a result, the above-described damage of the endless belt is more likely to occur particularly in a fixing device using such a relatively thin endless belt owing to the reduction in strength of the endless belt.